A. Field of the Invention
The present invention relates generally to a projection exposure system for transferring an image of a pattern onto a substrate, such as a semiconductor wafer. More particularly, the present invention relates to a projection exposure apparatus and method that provides enhanced precision in the alignment of a projection optical system and the substrate.
B. Description of the Prior Art
Semiconductor fabrication requires precise alignment of an optical system with a substrate in order to produce extremely detailed circuitry on the substrate. FIG. 1 illustrates a conventional projection exposure apparatus 10, which includes a light source 12, a stage 14, a lens column 16, a mask 18, and an interferometer 20. Light source 12 illuminates mask 18, causing a pattern from the mask to be projected through lens column 16. Lens column 16 projects the pattern onto a photosensitive substrate on stage 14. The optical axis of lens column 16 is indicated by the broken line down the middle of lens column 16.
Exposure of the pattern onto the substrate requires moving stage 14 to provide accurate alignment between the substrate and lens column 16. The optical axis of lens column 16 cannot be directly measured without disturbing the projection of the image of the pattern to stage 14. Therefore, in conventional systems, interferometer 20 detects alignment by projecting a first light beam 21 to a mirror on side 24 of lens column 16, and a second light beam 22 to a corresponding side 26 of stage 14. Each beam is reflected back through a beam splitter to a control device. As the beams travel back through the beam splitter they interfere with each other. Using the interference information, the control device determines the alignment between the optical axis and the substrate on stage 14. In response to the alignment determination, the position of stage 14 is adjusted in relation to lens column 16 to align the substrate on stage 14 with lens column 16.
Charges in beam 21, however, may not provide accurate information for alignment because of the sensitivity of lens column 16 to thermal expansion and displacement. Fluctuations in the air temperature may result in expansion of lens column 16, as generally illustrated in FIG. 2. Note that an even expansion (or contraction) of both sides does not displace the optical axis. FIG. 3 illustrates shifting of lens column 16. Shifting of lens column 16 results in the same reading from beam 21 as is received in FIG. 2 even though the optical axis in FIG. 2 has not shifted. Therefore, an adjustment of stage 14 based on the interferometer readings in FIG. 2 will result in an inaccurate alignment of stage 14 and lens column 16. Accordingly, errors arise in the exposure of stage 14 to the image of the pattern projected through lens column 16. More particularly, because conventional alignment procedures use only one side of lens column 16, all of the alignment problems are not properly detected. Accordingly, the adjustment of stage 14 in relation to lens column 16 is often inaccurate and, as such, results in errors in the projection of the image of the pattern onto the substrate on stage 14.
Further still, lens column 16 may be sensitive to vibrations in the projection exposure apparatus. Typically, adjusting stage 14 results in vibration throughout the projection exposure apparatus. This vibration may result in fluctuations of lens column 16, as generally illustrated in FIG. 3. Because of the fluctuations, the precise alignment of lens column 16 may no longer be attainable from the conventional alignment procedure. Accordingly, adjustment of stage 14 in relation to lens column 16 is often inaccurate and results in errors in the projection of the image of the pattern onto the substrate on stage 14.
Although these inaccuracies of conventional alignment procedures may be tolerable for the level of precision required under certain semiconductor manufacturing design constraints, the precision required in modern semiconductor manufacturing processes requires a reduction in or elimination of these inaccuracies. To attain such precision, errors caused by thermal deformation, displacement, and vibration in lens column 16 must be substantially eliminated.
Therefore, there is a need for a projection exposure apparatus and method that can substantially reduce inaccuracies in the alignment of the lens and the stage.